The present invention relates generally to processes for preheating oil shale to temperatures of about 550.degree. F. prior to pyrolysis. More specifically, the present invention relates to the preheating of oil shale by entrainment in a series of dilute phase lift pipes having gas streams of gradually increasing temperatures.
The present invention is an improvement upon U.S. Pat. No. 3,925,190 issued to Whitcombe et al. on Dec. 9, 1975. The patent issued to Whitcombe discloses an oil shale preheating process in which a series of dilute phase vertical transfer lines or lift pipes are used to preheat crushed oil shale to a temperature of between about 400.degree. F. to 650.degree. F. The Whitcombe patent and the disclosure therein are hereby incorporated by reference.
The preheating process as disclosed in the Whitcombe patent involves the use of at least two lift pipes and preferably three lift pipes which are serially connected together. Initially, crushed raw shale is heated to about 200.degree. F. by entrainment in a gas stream in the first lift pipe. The partially preheated shale is separated from the first lift pipe gas stream and passed to a second lift pipe where it is entrained in a hotter gas stream which raises the temperature of the oil shale to about 350.degree. F. After heating in the second lift pipe, the oil shale is again separated from the gas stream and passed to a third and final lift pipe where it is entrained in an even hotter gas stream. As the oil shale is entrained and lifted up within the third lift pipe, it is heated to its final preheat temperature of about 550.degree. F. This fully preheated oil shale is then separated from the gas stream in the third lift pipe and passed to a retort for pyrolysis.
The preheat process disclosed in the Whitcombe patent is designed for use in a pyrolysis system where the heat for pyrolysis is supplied by heat carrying bodies, such as ceramic balls, in a rotating retort. In this type of retorting process, the ceramic balls are heated in a ball heater, then transferred to the retort where they provide heat for pyrolysis with the cooled balls being passed back to the wall heater for reheating. The flue gas from the ball heater is a convenient hot entraining gas for use in the third lift pipe. This hot flue gas from the ball heater is typically at temperatures between about 1200.degree. F. and 1400.degree. F. As the partially preheated oil shale is contacted with this hot gas stream in the third lift pipe, a small amount of hydrocarbons, generally on the order of 500 to 1,000 ppm, are released from the oil shale in gaseous form and entrained in the flue gas. In order to prevent loss of the hydrocarbons originating from the oil shale, an incinerator is provided for combusting these entrained hydrocarbons after the flue gas stream is separated from the preheated oil shale in the third lift pipe. According to the Whitcombe patent disclosure, the incineration of the released hydrocarbons is carried out in an incinerator which is maintained at an incineration temperature of about 1400.degree. F. by combustion of a liquid or gaseous fuel and air.
The hydrocarbon free flue gas generated in the incinerator, is passed upstream for use as the entraining gas in both the first and second lift pipes. In this way, a convenient process is provided for utilizing the 500 to 1,000 ppm of hydrocarbons generated in the third lift pipe while providing a source of hot entraining gas for the first and second lift pipes.
At the flue gas temperature present in the first and second lift pipes, between 75 and 100 ppm of hydrocarbons are generated or released from the oil shale in gaseous form. The amount of hydrocarbons released may even be greater when higher flue gas temperatures are utilized. In the Whitcombe patent disclosure, the flue gas from the first and second lift pipes is simply vented to the atmosphere with no provision being made for utilizing the hydrocarbons entrained in the flue gas. This not only lowers the net hydrocarbon yield of the process, but in addition is environmentally undesirable and limits the temperature of entraining gases in the first lift pipe to relatively low levels.